In general, a high-temperature process and a high-vacuum process are essential for the formation of a semiconductor device using silicon, which is an inorganic semiconductor material, into a thin film. The high-temperature process is needed and hence silicon cannot be formed into a thin film on a plastic substrate or the like. Accordingly, it has been difficult to impart flexibility to a product into which the semiconductor device is incorporated or to reduce the weight of the product. In addition, the high-vacuum process is needed, and hence an increase in area of the product into which the semiconductor device is incorporated and a reduction in cost of the product have been difficult.
Under such circumstances, in recent years, research has been conducted on an organic semiconductor device utilizing an organic semiconductor material as an organic electronic part (such as an organic electroluminescence (EL) device, an organic thin-film transistor device, or an organic thin-film photoelectric conversion device). Such organic semiconductor material can markedly reduce a production process temperature as compared to the inorganic semiconductor material, and hence can be formed into a thin film on the plastic substrate or the like. Further, when an organic semiconductor having high solubility in a solvent and having satisfactory film formability is used, a thin film can be formed by an application method which does not require a vacuum process, for example, with an inkjet apparatus or the like. Consequently, the increase in area and the reduction in cost, which have been difficult in the case of the semiconductor device using silicon, which is the inorganic semiconductor material, are expected to be realized. As described above, the organic semiconductor material has advantages in, for example, the increase in area, the flexibility, the reduction in weight, and the reduction in cost as compared to the inorganic semiconductor material. Accordingly, the organic semiconductor material has been expected to find applications in organic semiconductor products taking advantage of such characteristics, e.g., information tags, large-area sensors such as electronic artificial skin sheets and sheet-type scanners, and displays such as liquid crystal displays, electronic paper, and organic EL panels.
The organic semiconductor material to be used for the organic semiconductor device expected to find a wide range of applications as described above is required to have a high charge mobility. For example, in an organic transistor, the charge mobility directly affects a switching speed and performance of an apparatus to be driven, and hence an improvement in charge mobility is an essential issue in achieving practical use. Further, as described above, in order to enable production of a semiconductor device by the application method, the organic semiconductor material is required to have solvent solubility, oxidation stability, and satisfactory film formability.
The high charge mobility is particularly mentioned as a characteristic required of the organic semiconductor. From this viewpoint, an organic semiconductor material having a charge-transporting property comparable to that of amorphous silicon has been reported in recent years. For example, the same level of charge mobility as that of the amorphous silicon has been reported in an organic field-effect transistor device (OFET) using, as an organic semiconductor material, pentacene, which is a hydrocarbon-based acene-type polycyclic aromatic molecule in which five benzene rings are linearly fused (Non Patent Literature 1). However, the use of pentacene as an organic semiconductor material for an OFET is disadvantageous from the viewpoints of an increase in area, flexibility, a reduction in weight, and a reduction in cost because an organic semiconductor thin-film layer is formed by a deposition method in an ultrahigh vacuum. In addition, there has been proposed a method of forming a pentacene crystal in a dilute solution of trichlorobenzene without employing a vacuum deposition method, but the production method is difficult and hence a stable device has not been obtained yet (Patent Literature 1). The fact that the hydrocarbon-based acene-type polycyclic aromatic molecule like pentacene has low oxidation stability has also been pointed out as a problem.
In addition, a polythiophene derivative having a long-chain alkyl group, such as poly(3-hexylthiophene), is soluble in a solvent, and its use in production of an organic semiconductor device by the application method has been reported. However, there has been a problem in that its charge mobility is lower than that of a crystalline compound, and hence characteristics of the resultant organic semiconductor device are low (Non Patent Literature 2).
In addition, pentathienoacene, in which thiophene rings are fused, is improved in oxidation resistance as compared to pentacene. However, pentathienoacene has a low carrier mobility and requires many steps in its synthesis, and hence has not been a material preferred for practical use (Non Patent Literature 3).
In addition, recently, there has been a report of an extremely high mobility achieved with a single crystal of rubrene, which is an acene having high solubility. However, a film of rubrene formed by solution casting does not adopt such single-crystal structure, and does not provide a sufficient mobility (Non Patent Literature 4).
As examples of a hydrocarbon-based acene-type compound having high solvent solubility and being relatively stable against oxidation, some compounds each obtained by substituting the 6- and 13-positions of pentacene with silylethynyl groups have been reported to provide coating films having good stability (Non Patent Literature 5). However, in such report, a qualitative property, i.e. an improvement in stability against oxidation is only mentioned, and stability sufficient for practical use has not yet been obtained.
Meanwhile, a heteroacene-based skeleton obtained by introducing a heteroatom, such as nitrogen or sulfur, into a hydrocarbon-based acene-type polycyclic aromatic skeleton has recently been reported. However, its characteristics are not sufficient, and for example, in the case of an indolocarbazole-based material obtained by introducing nitrogen as the heteroatom, a sufficient charge mobility has not yet been obtained (Patent Literature 2).